Proper way to check type when object might be an object() instance

Question

I'm looking for the proper way to check a type of object that might be an instance of object() now i'm doing this:

def _default_json_encoder(obj):
    """ Default encoder, encountered must have to_dict
    method to be serialized.  """
    if hasattr(obj, "to_dict"):
        return obj.to_dict()
    else:
        # object() is used in the base code to be evaluated as True
        if type(obj) == type(object()):
            return {}
        raise TypeError(
            'Object of type %s with value of '
            '%s is not JSON serializable'
            % (type(obj), repr(obj))
        )

but

if type(obj) == type(object()):
    return {}

is not recommended, unfortunately

if isinstance(obj, object):
    return {}

won't work because all object will be evaluated as True.

So type(obj) == type(object()) is the only way ?

Why do you think you need to test for object instances? Everything in Python 3 is an instance of (a subclass of) object. Do you have actual object() instances in your data, and if you do, why? — Martijn Pieters
– Martijn Pieters, Commented Sep 30, 2016 at 10:25
And the usual way to test for a class is to use isinstance(obj, class), but again, for the object class, that's everything. You can use type(obj) is object to disallow any subclasses. I'm puzzled as to why you'd need this however. Why not just use True or some dedicated class instead? — Martijn Pieters
– Martijn Pieters, Commented Sep 30, 2016 at 10:25
@MartijnPieters, because in some case the I have made affectation with object() then i need to repr this (instance attribute) as dict. — Ali SAID OMAR
– Ali SAID OMAR, Commented Sep 30, 2016 at 10:32
But why? What is the actual problem you tried to solve by using object()? — Martijn Pieters
– Martijn Pieters, Commented Sep 30, 2016 at 10:33
I answered in my previous comment. the need to repr an instance attribute as {}. — Ali SAID OMAR
– Ali SAID OMAR, Commented Sep 30, 2016 at 10:36

Martijn Pieters · Accepted Answer · 2016-09-30 10:31:29Z

1

If you really have instances of object() in your data structure, then you'd use:

type(obj) is object

to test for direct instances (classes are typically singletons).

However, I'd rethink my data structure, and either use another primitive value (like True), a dedicated singleton, or a dedicated class.

A dedicated singleton could be:

sentinel = object()

then test for that sentinel:

if obj is sentinel:
    return {}

answered Sep 30, 2016 at 10:31

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ali SAID OMAR Over a year ago

As usual nice response, Thanks !

Collectives™ on Stack Overflow

Proper way to check type when object might be an object() instance

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related